1. Field of the Invention
The technical field relates to time-delay and integration (TDI) charge coupled device (CCD) sensors, and, in particular, to sensors with reticulated gate CCD pixels and diagonal strapping.
2. Description of Related Art
When CCD sensors are operated in short wavelengths of ultra violet light, referred to as deep ultra violet (DUV), different regions of a CCD pixel have different responses to illumination. For example, polysilicon in regions covered with polysilicon will effectively absorb all incident DUV illumination so that no photon generated signal electrons are produced, resulting in areas of low quantum efficiency for DUV illumination. Conversely, regions covered with field oxide, or regions that are formed into photodiode or pinned photodiode, will permit DUV photons to enter a substrate and create photoelectrons that can be integrated to form signal charge, introducing areas of high quantum efficiency. The problem of variation in illumination responses by regions also occurs, to a lesser extent, with visible illumination, since polysilicon also has some absorption of visible light photons.
FIG. 1 illustrates a prior art device with a single CCD pixel 50 that includes a localized region 3 of high quantum efficiency. The localized region 3 of high quantum efficiency is formed, for example, by creating a pinned photodiode region adjacent to a vertical CCD (VCCD) channel 9. This pixel architecture is often referred to as a “reticulated gate”. In a conventional TDI imager, polysilicon CCD electrodes are continuous stripes across arrays of pixels. Here, these electrodes are “reticulated” over a portion of each pixel by deliberately removing polysilicon from areas of the pixel outside the VCCD channel 9, to enable localized regions of high quantum efficiency (e.g., region 3) to be formed as a photodiode or pinned photodiode.
Localized regions 3 of high quantum efficiency are disposed in rows and columns of a TDI array, as shown in FIG. 2. FIG. 2 is a schematic illustration of a group of individual pixels 50 of FIG. 1 organized into a TDI array to be used in TDI CCD sensors. The pixels 50 are spaced vertically and horizontally by a pixel pitch P. In this example, the TDI is in the vertical direction, pointing downwards toward the lower edge of FIG. 2.
FIG. 3 is a graph showing a net TDI response of the arrays of pixels 50 in FIG. 2. The TDI pixels 50 have localized regions 3 of high quantum efficiency, and regions between regions 3 have a comparatively lower response. The net TDI imaging response has peaks localized to these regions 3 of high quantum efficiency. The CCD pixel 50 has greater sensitivity to sense light at specific locations within the pixel 50. When an image contains patterns (e.g., stripes) at high spatial frequencies, for example, spatial frequencies that define patterns that repeat more often than the pixel pitch (i.e., above Nyquist rate), and a fill factor of the sensor pixels is less than 100 percent, spurious shapes of the resulting image typically appear, a phenomenon creating aliasing. Aliasing can be reduced by blurring the image, hence reducing the high spatial frequencies in the image. Blurring is typically achieved by lens defocusing or with an anti-aliasing filter, either optically or electronically.
One approach for reducing aliasing, and consequently, enhancing modulation transfer function (MTF), is to position a shift region (i.e., a jog region) in the array of pixels 50 to maintain uniformity of response. The localized region 3 of high quantum efficiency is shifted by a fraction of one pixel pitch, so that the jog region enables a whole column of pixels 50 in the TDI array to generate a more uniform level of response, resulting in enhanced MTF.
FIG. 4 is a schematic illustration of a group of pixels 50 of FIG. 2, with a lateral position shift region halfway down a column of the array. Referring to FIG. 4, one-half of the pixels 50 in the array have a lateral shift by ½P midway down the array. As a result, the lateral shift creates a more uniform net TDI response even with a localized area of enhanced quantum efficiency or photoresponse within each pixel 50. FIG. 5 is a graph showing the net TDI response of the columns of pixels 50 in FIG. 4, where the imaging response is no longer localized to specific lateral areas.
In another aspect of TDI CCD sensor design, metal bussing is introduced into the array of pixels 50 to reduce resistance capacitance (RC) time constants of polysilicon clocking, and to maintain a high fill factor of the TDI imaging region. FIG. 6 is a schematic illustration of the group of pixels 50 of FIG. 2, with a single metal bus 25 overlaying the array of pixels 50, but without MTF-enhancing offset in pixel position. One metal bus 25 is shown for purposes of illustration.
FIG. 7 is a graph showing the net TDI response of the columns of pixels 50, where the imaging response is localized to specific lateral areas (as in FIG. 3), but has been reduced slightly in all pixel columns by the shadow effects of the metal bussing. However, the localized response is still present in this example, as in FIG. 3.
The benefit of MTF-enhancement architectures and metal bussing structure can be combined by positioning the jog region (as in FIG. 4) and metal bussing (as in FIG. 6) into a single TDI array. However, the addition of opaque metal bussing to a TDI array using MTF-enhancing offsets in pixel position introduces pixel response nonuniformity (PRNU). Specifically, the localized regions 3 of high quantum efficiency within a pixel 50 may or may not be covered, depending on the specific location of metal bussing in the array. Therefore, vertical columns may not have the same effective fill factor in the DUV, creating nonuniformity of response between columns. PRNU is demonstrated in FIG. 8 and FIG. 9.
FIG. 8 is a plain view of an array of pixels 50 of FIG. 4, with MTF-enhancing architectures, and with metal bussing structure of FIG. 6. The metal bus 25 crosses dissimilar portions of the pixel 50, depending on where the metal bus 25 is positioned in the array, creating asymmetric response. FIG. 9 is a graph illustrating the net TDI response of the array of pixels in FIG. 8, showing a dissimilar response between the two regions of the array.
Since a CCD typically has many clock phases requiring many metal busses 25, and multiple lateral shifts of pixel position may be needed to enhance MTF, a complex and irregular response will likely be introduced. One possible solution is to create layout geometries where the metal busses 25 occupy the same portion of each pixel. However, this solution requires complex layouts to shift the metal busses 25 across the array in such a way that all TDI columns are affected equally.